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How to Finish Exterior Foundation Insulation

Options abound for covering up above-grade rigid foam insulation, including one that may surprise you

Posted on Nov 15 2010 by Scott Gibson

Energy gurus and building codes routinely recommend these days that foundation walls be insulated. One way of accomplishing that is by adding a layer of rigid foam insulation on the outside of the foundation.

And that's exactly what William Poole is planning to do.

Most of the rigid foam insulation will be underground and out of sight. But what do you do with that stretch of exposed insulation above grade?

"I just can't get excited about options for covering the insulation in the space between the top of grade and the bottom of the siding," Poole writes in a Q&A post. "Has anybody completed a project with this detail and come up with a material that is both practical and, for lack of a better term, 'good looking'?"

Here's a list of options
GBAGreenBuildingAdvisor.com senior editor Martin Holladay is first out of the box with a list of not just a few but 10 potential solutions.

He lists four possible generic products: pressure-treated plywood, fiber-cement panel siding, stucco, and surface-bonding cement. There also are a number of proprietary products worth looking at, Holladay says, including Insul-Guard 2 fiberglass from Diversified Composites, Ground Breaker fiberglass covering from Nudo Products, FP Ultra Lite Panels from Styro Industires , and Protecto Bond membrane, a peel-and-stick product from Protecto Wrap.

Robert Riversong has used surface-bonding cement with "excellent success." He adds extra acrylic mortar modifier to the mix to improve plasticity, adhesion and water resistance. And for a "belt-and-suspenders" approach, he runs 1/2-in. hardware cloth from the sill to a few inches below grade before troweling on the cement.

"It leaves a highly water-resistant and very tough 'stucco' coating that looks great and has excellent tensile strength and crack-resistance because of the fibers," Riversong writes.

The coating should be vapor permeable even with the acrylic additive, he adds, and can be waterproofed with UGL Drylok latex masonry sealer, available in both gray and white.

Fiber cement has some caveats
Allison Bailes writes that he's heard recommendations for fiber cement, but adds, "I always worry about that. I've seen scrap fiber-cement siding lying on the ground and delaminating after getting wet. Yes, if the water is managed correctly, it should never be wet for very long, but still, it's outside with the bottom in the dirt.

GBA Advisor Michael Chandler, a builder in North Carolina, uses Hardie backer made by James Hardie. Joints between panels are covered with fiber mesh tape embedded in surface-bonding cement, then covered with a sponge finish stucco and elastomeric paint.

"We didn't have any issues with delamination of the Hardie-backer, but we had to go in within a year and caulk all the vertical cracks and re-paint," Chandler says. "Since then we've been fine, but on other projects where we stucccoed over mechanically fastened Spider Lathe synthetic stucco lathe we've not had as much cracking."

Allison's concerns are well founded, says Holladay. Drawing on research he originally did for Energy Design Update, Holladay says only one fiber-cement manufacturer, MaxiTile of Carson, California, allows installation in contact with the ground.

James Hardie, Cemplank and CertainTeed all discouraged that application.

Mark Hinrichs adds that prefinished fiber-cement panels and boards and cement backerboard can be used in contact with the ground, but only in mild climates.

"But the manufacturers don't recommend them for colder climates where they will be subjected to freeze-thaw cycles," he adds. "They will begin to deteriorate after 100-150 freeze-thaw cycles. This from the James Hardie engineering department when I called to ask."

Other options
Although Riversong has had good luck with surface-bonding cement, he points to a way of sidestepping the protection issue altogether by using a Thermomass foundation.

"Another alternative, to avoid the problems of external foam board, is to build on a Thermomass foundation, which places the XPS midline in the concrete wall, with the inner and outer wythes connected 12 in. o.c. with fiberglass ties that have the tensile strength of ½-in. rebar," he says.

The foundation gets the same insulation benefit, but now the foam is out of harm's way.

And the solution that may surprise you?
"Try a spray on bed-liner application," writes NH Green Builder. "Some guys are set up to do it mobile, although it is fairly expensive it looks great."

"The idea to cover it with a spray-on bed-liner is really interesting," says Max York. "Has anyone else tried that? It doesn't sound great for the environment though."

Our expert's opinion
Here's how GBA technical director Peter Yost responds:

First, a confession: Steve Baczek, Mike Guertin and I missed this issue when we were doing the Key Materials tips for the GBA foundation construction details involving exterior rigid insulation for basement and crawlspace foundations. We did include a tip on the protective covering for slab foundations but it’s not nearly as helpful as the contributors to this Q&A post.

Second, this detail has always troubled me both aesthetically and functionally. It’s a little bit like wearing your pants below your drawers (here, I date myself, I realize… ). But we have to keep the finished grade at least 6 to 8 in. down from framed above-grade assemblies by code, so we end up with a strip of unfinished structure exposed (and on steeply sloped sites, sometimes a lot more than a strip). Back when I actually built stuff, we always applied a parge coat of latex-modified stucco with joints taped with fiberglass mesh. But it was a bit of handwaving at both the aesthetics and the function; it did not look that great and was not very durable.

I think the best looking option, but certainly not the least expensive, is cultured stone veneer. Its greater thickness is quite likely to mean a water-table detail at the transition to the above-grade wall. And for any exterior foam insulation more than ½ in. thick, you are likely to need special fasteners securing the insulation to the basement foundation wall, or even a brick ledge. The stone veneer turns this transition from a bit of an aesthetic cluge to an architectural feature. Beautiful, but pricey.

Methods described by Martin, Robert, and Michael all made sense to me. I agree that while a material like Hardie backerboard can work (and I like protective coverings that are essentially hard sheet goodsMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen., as more robust in protecting the insulation), fiber-cement siding materials are not a good fit.

In terms of vapor permeability: many coating manufacturers can now pretty much “dial in” this property, even for elastomeric coatings; just get the standardized test numbers from the manufacturer. But whether or not you need drying to the exterior depends a lot on drying potential to the interior. Concrete foundation walls don’t do much drying to the exterior, except above grade.


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  1. Green Building Advisor

1.
Mon, 11/15/2010 - 15:56

Energy Edge
by Armando Cobo

Helpful? -1

Here is an awesome product for your foundation: www.energyedgeform.com. They are from Wichita, KS, but its my understanding they are in the process to have their product distributed by a national chain. You can contact Tom Campton there and he'll work with you. I highly recommend it.


2.
Mon, 11/15/2010 - 16:41

Response to Armando
by Martin Holladay, GBA Advisor

Helpful? -1

Armando,
The product you link to appears to be intended for new-construction slabs -- not for basement foundations or retrofit insulation.


3.
Mon, 11/15/2010 - 16:45

Metal?
by jeff_williams

Helpful? 0

What about metal cladding? I've seen a home in N MN use a metal cap (not sure if aluminum or stainless) on the top of the insulation with the sheeting held on with metal straps and then another sheet of metal slid up under the cap the continues below grade by a bit. It is in a sand/gravel soil situation.


4.
Mon, 11/15/2010 - 17:29

Energy Edge for retrofits
by Armando Cobo

Helpful? 1

Martin, They do have a retrofit and top of basement walls products as well. They have a pretty extensive list of drawings on their wesite, but let me contact them and ask them to post some drawings or pics or if you have anything more specific.


5.
Mon, 11/15/2010 - 19:14

Fastening PT plywood
by Lucas Durand

Helpful? 0

Martin, (if you're reading) how would you fasten PT plywood to a concrete wall through 4-6 inches of EPS or possibly rock-wool?


6.
Mon, 11/15/2010 - 20:12

Lucas The best way that I
by Garth Sproule

Helpful? 0

Lucas
The best way that I know off is to include the PT plywood and the insulation as part of the outside forms for your concrete pour. Simply run some long screws or nails right through the assembly and into the space where the concrete will be. Obviously, this would not work as a retrofit.


7.
Mon, 11/15/2010 - 20:29

Hmm
by Lucas Durand

Helpful? 0

Thanks Garth. That definately sounds like the easiest way. The PT plywood would only extend say, 24" below grade (while the insulation continues to the footing). Could the formwork be jigged to accomodate an extra 1/2" of plywood at the top?
Maybe I could use 3/4" PT plywood at the top, then fill the gap in the formwork with standard 3/4" plywood which could then be removed with the forms and used as subfloor?


8.
Tue, 11/16/2010 - 00:48

Have you had a good look at
by Garth Sproule

Helpful? 1

Have you had a good look at the thermomass system? They will sell their proprietary spacers to DIY'ers...


9.
Tue, 11/16/2010 - 06:27

Metal for retrofit
by Rick

Helpful? 1

Jeff,
I would be more than a little surprised if it was stainless.
I have used 20 ga. aluminum and it has done well for 2 years so far. The house started out with 6-12 in. band of foundation above grade, aluminum siding over 1 in. foam over studs with steel let-in bracing. Sheet metal shop bent 10-ft. long Z sections. One-inch vertical leg gets nailed to the bottom 2-by plate, through the 1 in. foam. Middle, "horizontal" leg that is not horizontal covers the thickness of foundation foam, and the last leg extends well into the ground. Just needed hand brake and shears to work the corners in the field. The painted aluminum is complimentary to the siding and is holding up very well against the weed eater. Automatic termite shield is built in. More, added layers of exterior wall foam with furring strips extend down to the top of the foundation cladding. Screened venting between furring strips is hopefully using the chimney effect to draw off some summer heat from the claddilng. Disadvantages are minor to me: Relatively small thermal bridge, the aluminum will dent easily. If I had it to do over again, I might choose a lighter color for the foundation cladding---I have wondered, even with the venting, if my xps under the cladding is getting a little hot on the south side.


10.
Tue, 11/16/2010 - 07:48

Response to Lucas Durand
by Martin Holladay, GBA Advisor

Helpful? -1

Lucas,
Although I have interviewed builders who have protected rigid foam with PT plywood, I have never done it myself, so I'm not sure which type of fasteners are best. In addition to a few fasteners, it's always possible to use an adhesive.

If I had to do it, I would call up some manufacturers who specialize in masonry and concrete fasteners and ask for the best fasteners for this application.


11.
Tue, 11/16/2010 - 12:29

Insulation
by Perry525

Helpful? 0

I imagine that the purpose of the insulation is to help keep the home warm?
With this in mind and as heat always moves to cold.
Surely the logic is to have the insulation as close to the comfort zone as possible?
Therefore, the insulation needs to be tight under the floor.
What point is there, in letting your heat escape into the crawl space, where much diluted it is very slightly slowed on its journey to the sky?


12.
Tue, 11/16/2010 - 13:21

Response to Perry 525
by Martin Holladay, GBA Advisor

Helpful? 0

Perry525,
There are several reasons why it might be desirable to insulate a crawl space wall or a basement wall instead of the floor above:

1. If it is a basement, it might be desirable to heat the space, since many homeowners use basements for living space.

2. If it is a crawlspace, insulating the walls prevents the pipes from freezing.

3. Most crawl spaces include heating ducts. Insulating the crawl space walls brings the heating ducts inside the conditioned space of the home, saving energy.

4. When crawl space walls are uninsulated, the crawl space is often moldy.


13.
Tue, 11/16/2010 - 14:35

EnergyEdge Insulates, Protects & Finishes Concrete Edges
by R. Tom Compton, AIA

Helpful? -1

EnergyEdge is a product that has specifically been designed to address this exact problem. The transition from below grade to above grade insulation systems has traditionally left a gap in the buildings thermal envelope. We have photos and details of installations, both for new construction and retro-fit applications, for slab-on-grade and basement construction. For slab edges, our product actually functions as the form, insulation, protection & finishing system, saving time & wood resources while reducing construction waste.

We are not a new company but are new in the CertainTeed distribution network. We will be on exhibition with them at the USGBC Green Build Show in Chicago beginning tomorrow. Please visit either of our web pages, www.EnergyEdgeForm.com or www.CertainTeed.com/products/foundations , for information. You can also contact your local CertainTeed sales rep. If you would like for us to send you specific information on your application, please contact Casey Barbour at CEB@EEForm.com .


14.
Wed, 11/17/2010 - 06:30

Terne-Coated Stainless
by Doug

Helpful? 1

TCS might be a better metal for these types of details, compared to aluminum.
In addition to being much stronger, therefore more likely to look decent after a few decades of bumps and weedwacking, it also has a lot lower thermal conductivity.
Further it's much more resistant to corrosion from alkaline masonry products.
I still find this part of a house to be a real head-scratcher. Well over half the houses I've renovated have termite damage, and covering the foundation up to the framing seems like an open invitation. But, running something termite-resistant and durable through the insulation creates such a thermal bridge that it seems questionable also.
Anyone tried vinyl trim coil? (I know, I know...but if durable, thermally low-conductive, and prevents future use of termiticides might be a net benefit.)


15.
Wed, 11/17/2010 - 06:34

TCS more
by Doug

Helpful? 0

Forgot to mention that in our area, better roofing companies often have a leftover coil of TCS sitting in their shop. It's used as a high-end roofing. Two years ago when copper pricing went nuts, it became less expensive than copper and is probably more durable.
It's only a little bit harder to work compared to copper, a bit harder to bend and a bit harder to cut.
We get essentially comparable pricing to copper, so we have been using it for flashings quite a bit, anywhere that copper's color or eventual verdigris dripping would be less preferred to the soft gray color of TCS.


16.
Wed, 11/17/2010 - 07:29

Response to Doug
by Martin Holladay, GBA Advisor

Helpful? -1

Doug,
One of the products I suggested, Insul-Cap, is a vinyl product made to protect foam insulation. It is manufactured by the Wisconsin Poured Wall Contractors Supply, a company that does not maintain a Web site.

Here is a Web page that provides contact information for the company:
http://www.akama.com/company/Wisconsin_Poured_Wall_Contractors_Supply_af...


17.
Wed, 11/17/2010 - 17:33

Galvanized roofing
by Allan Bullis, CEM LEED AP

Helpful? -1

I have recently used standing seam coil stock provided by a local roofer. It comes in multiple colors, and can be bent to tuck under the siding with a break. I also hemmed the overlaps or where butting up to items like an exterior chimney. This reduces the 'oil canning' that can occur. To secure, used PL 200 construction adhesive. It fast to install and looks great!!


18.
Wed, 11/17/2010 - 17:46

How to Finish Exterior Foundation Insulation
by Bruce Wilson LEED AP

Helpful? -2

We just finished a job. The foundation is block on an addition we are just finishing. It was covered with an inch and a half of Foamular foam by the foundation contractor as per the drawing specs. We used the acrylic modified base coat used with Dryvit with a fiberglass mesh emebedded in it. Then top coated with Dryvit. Our local Dryvit dealer OK'd the installation.
We use rigid insulation outside of the foundation and outside of the sheathing to prevent thermal bridging. One of my workers calls it outsulation instead of insulation. I have had good success with this and my clients like the added comfort with no air leaks. All the seams get taped and the window flanges get taped to the foam. I use cellulose insulation in the walls and roof.


19.
Wed, 11/17/2010 - 21:04

Termites in Heaven
by Tom Southern

Helpful? 1

Guys, the solutions I've read here, and the detail sketched in the article all share one serious drawback, I'm all with you about insulation, but I'd be scared to death to not have some way to keep an eye out for termites coming up into the house from the crack between the foundation wall and the foam board. The termites will be thanking you till hell freezes over for giving 'em the 'cover of darkness'. Believe me, I want to be wrong because I'm a tree hugging environmental carpenter and builder and that goes hand in hand with going all out on insulation, but.........


20.
Thu, 11/18/2010 - 08:23

Foundation cover
by Carl Mezoff

Helpful? 1

Here is a vote for galvanized wire lath nailed with large washers through the foam into the CMU, and then parged over with mortar. A strip of aluminum coil stock under the PT sill plate and extending out over the foam has (as far as I can tell) prevented termite attack (in our house here in Connecticut) for the last 30 years. There are a few hairline cracks in the parget because this was done prior to the availability of acrylic latex additive which I would add to the stucco/mortar mix if I were doing it today.


21.
Thu, 11/18/2010 - 09:37

Continous Insulation Solution
by Randy Wilson

Helpful? 0

We suggest using ProGuard Cement Faced Insulated Sheathing. This material is rigid insulation with a cement board facing. The advantages include: continuous insulation, durable substrate, simple installation (Fomo adhesive and screws), waterproofing membrane over all attachments, perfect substrate for thin brick, thin stone and stucco, 15 year system warranty and is manufactured in USA Midwestern states (as opposed to overseas for some of the products mentioned herein). You can find more information at www.centerlinematerials.com


22.
Thu, 11/18/2010 - 11:17

Styro Industries
by Jason Hackworth

Helpful? -1

We've just started using Stryro Industries Tuff II trowel on coating which was recommended by Dow. It's very durable and fairly easy to apply.


23.
Thu, 11/18/2010 - 11:39

Insulation for pier built home
by Barry Sheriff

Helpful? 0

I recently bought a log cabin built on cement piers in N. H. There is an opening of about 18 inches between the ground and the bottom log. I had new unfaced insulation put in between the floor joists and plywood put under that. I would like to put a skirt around the house from the ground to the bottom log (should I go below ground level). Should I use plywood with a blue board backer? Obviously, I know very little about this and I am getting different suggestions from each person I talk to.


24.
Thu, 11/18/2010 - 11:53

Response to Barry
by Martin Holladay, GBA Advisor

Helpful? -1

Barry,
A durable skirt can be built using pressure-treated 2x4s and pressure-treated plywood. Just build a mini-wall. If you want to keep the crawl space warm, sandwich some rigid foam between the stud wall and the PT plywood.


25.
Thu, 11/18/2010 - 13:14

Exterior insulation benefit vs cost
by Don Otto

Helpful? -1

While exterior rigid foam insulation has its benefits, it's really expensive to protect, regardless of how you do it. EIFS, with the fiberglass mesh is expensive and not that durable. T-Mass is about 1.5 times as much as spraying closed-cell urethane foam on walls furred and insulated inside. (Joe L's objections notwithstanding.) I agree that stone or masonry is esthetically pleasing, and it's wonderful if the owners want to afford it, but is still one of the most costly. I have a hard time justifying the benefit of exterior foam when what we're after is saving energy and providing comfort. While I hold my nose in recommending interior framed walls for insulation, in my eastern Iowa location it's the best value to the client.


26.
Sun, 11/21/2010 - 22:24

Basement Insulation
by Doug Colby

Helpful? 0

I have an insulation company in N.H. and when I go over options for my customers we usually opt for spray foam insulation on the interior of the foundation.
Here are my reasonings
1. We can do a thermal barrier that is maintenance free inside the home and dont have to worry about someone backfilling against our insulation and damaging it.

2. As one other poster stated if there is no break of the rigid insulation you are inviting a problem with termites or ants or whatever. There is no food value to the foam but it is a very good home for insects.
3. As a minimum value is the fact that the rigid hast to go on as soon as possible with spf it can be applied later and if any cracks in basment wall happens we can spray and them while not a garunteed water stopper it is pretty good.

4. last one that comes to mind is the spf is done after everyone else mechanical or plumber or whoever and we can make sure the penetrations through the envelope are sealed properly.

Generaly speaking it seems like most people want to do it yourself which is great but a general contractor is under the gun to meet budget and time and the exterior insulation is good but it seems to me alot of maintenance down the road and potential for problems.
That what i think lol Doug


27.
Mon, 11/22/2010 - 09:53

In 1982 I built my 2 story
by Ernest G. Mueller

Helpful? 0

In 1982 I built my 2 story house with outside insulation from the foundation to the roof line. The 1 inch insulation at the outside of the block wall is covered with embossed aluminum sheets. After 28 years it still looks great. The upper walls have vertical 1x4 T&G western cedar siding over the outside insulation. I am very pleased with this design.


28.
Mon, 11/22/2010 - 12:22

Question and comments
by Matthew Klein

Helpful? 0

I have a question about the detail drawing in the article. A line, which represents some kind of material is shown covering the insulation and extends from below the sill plate to below grade. Everything else in the wall structure is called out except that item. I am assuming that it represents the hardware cloth discussed in the article. Am I right?

I also noticed that flashing is not shown over the top of the insulation in about the same location as the item that is not called out. I am thinking that flashing would be needed to prevent water penetration between the insulation and concrete. Such flashing could also act as a termite shield listed as a concern in the messages.

After reading all of the discussions, I see no easy answer to this issue and this article and comments pushed that boat farther from the shore. In our area, I would say our three main concerns are freeze/thaw, weed-whackers and moisture. I can see using metal panels over the insulation as long as it goes below frost grade. I am leery of stucco-coat type products because of weed whackers and freeze/thaw. PT plywood is a no-no because, over time, the treatment leaches out of the wood and exposes the wood to decay and insects. Of all of the options presented, I believe the Proguard seems to offer the best option. Its product information says it can be used below grade, although I would prefer to see more assurance than a line or two in the product literature. If this product could be used as the exterior insulation, that would be ideal.

Still, in our area, insulation is not installed often on the exterior foundation other than concrete insulated forms. Basement foundations are usually insulated from the grade line up to the sill plate on the inside rather than outside. Relatively rarely do I find insulated crawlspace foundations. So, we are pretty much left with the CIFs for now, and I am not really crazy about the stucco finish usually used on them; but time will tell about the durability.


29.
Mon, 11/22/2010 - 12:35

Repsonse to Matthew Klein
by Martin Holladay, GBA Advisor

Helpful? 1

Matthew Klein,
I agree with you -- it is best to include metal flashing that extends from the gap between the mudsill and foundation, out over the protective covering installed over the foam. The flashing should have a 45-degree lip (drip edge) to kick out any moisture away from the house.


30.
Mon, 11/22/2010 - 17:25

I have used copper sheet for
by brian

Helpful? 0

I have used copper sheet for this detail.It ages to a nice patina that seems to blend in between ground and wall finish and it will probably outlast any surface-applied cement finish.A bit pricey for a detail people tend to ignore.but it is worth it to me to avoid call-backs.


31.
Mon, 11/22/2010 - 20:10

ICF manufacturers appear to recommend parging cement
by tim Rowledge

Helpful? -1

According to at least a few manufacturers that I looked at before using ARXX ICF blocks for my house, a favoured solution is parging cement to 6" below grade. I haven't got to the stage of needing to apply it yet and given the sudden appearance of snow I might not get there 'til next spring!


32.
Tue, 11/23/2010 - 15:57

Need steps laid out on how to insulate exterior foundation
by Steve Mattera

Helpful? 0

Guys I could see the solution for my application from your notes, some partial, some not. Here is the problem, I understand there is great benefit to insulating the exterior foundation that is above grade.

1. it is a 100+ yo two story wood framed on a rubble foundation. We are in northern VT (so cold climate)
2. vinyl siding comes down to about 8-12” from the top of finished grade.
3. the sill looks to be in decent shape but it not airtight on the foundation.
4. the exterior foundation face is a mix of rubble stone, faced with some stucco. Lots of stucco has broken and crumbling, as is some of the rubble.
5. I like the suggestion of facing it with cultured stone – but how? How I can reasonably secure lath to the loose crumbled areas of the wall, strapping? In this application would you remove the old loose masonry, repair it, cover it with blue board insulation, cover the insulation with lath, fasten the lath through the blueloard and deep into the masonry ? if so, using what fasteners? And would this require Z flashing from behind the siding over the top of the stone?
6. How about blueboard with PT sheets and lath and parge right to that?


33.
Tue, 11/23/2010 - 16:45

Response to Steve
by Martin Holladay, GBA Advisor

Helpful? 1

Steve,
For a rubble foundation, I think your only choice is closed-cell spray polyurethane foam, followed by stucco. It would also be possible, but more expensive, to prepare the cured foam for adhered manufactured stone veneer.


34.
Tue, 11/23/2010 - 22:29

Response to Martin
by Steve Mattera

Helpful? 0

Martin, thanks. In that solution, stucco directly to the foam (no board, no lath, etc)? Any Silpro products come to mind to address this?


35.
Wed, 11/24/2010 - 04:32

Second response to Steve
by Martin Holladay, GBA Advisor

Helpful? -1

Steve,
The synthetic stucco used for EIFS is applied directly to EPS foam, but most EIFS systems require fiberglass netting as well. Traditional stucco requires wire lath, which would need to be mechanically fastened somehow.

Your best bet would be to consult with a stucco contractor or an EIFS contractor.


36.
Wed, 11/24/2010 - 22:25

Skirting
by gary mccallum

Helpful? -1

I've done this several times and just used aluminum off a gutter machine roll. You have chouce of colours and can just nail it to the cantalivered framing. It is still working on a passive solar house I built in 1981


37.
Fri, 11/26/2010 - 18:13

stucco
by ecoralph.blogspot.com

Helpful? 1

I think SBC would probably work better, but it is harder to come by (and probably more expensive).
I've done a parge coat with type N cement and soft sand, and I've also tried a mix with straight portland. The type N gives a lighter-colored finish, and doesn't seem to get shrinkage hairline cracks as much as the portland. It's not as strong as the portland mix though, so it needs to be thicker to avoid cracking it if you bump it with a lawnmower.
The foam needs to be clean, or the stucco will eventually flake off; when exposed for a couple months the surface of the polystyrene degrades to a yellowish powder, which leads to poor adhesion of the stucco and flaking off.

A local EPS manufacturer also sells a product specifically for parging over foam.
http://www.truefoam.com/en/home/insulationproducts/forfoundations/pargep...


38.
Sun, 11/28/2010 - 20:51

Protecting exposed rigid foam with parging
by aaron

Helpful? 0

20 years ago, I had my exterior rigid foam insulation parged. The parging went over stucco wire. It is still hanging on. This is in Edmonton, Alberta.


39.
Wed, 12/14/2011 - 23:08

agree with Tom Southern
by Gred Gross

Helpful? 0

Don't know if that's your real name, Tom, but, yes, here in the south we do have a termite problem. Ants, too. They don't eat the foam but they sure like living in it. I've done a number of foundation (not lately) where we parged the foam down to the footer with a finish coat above grade. We've done it with and without reinforcing mesh. The latex modified stucco sticks pretty good to a roughed up surface.

It's a lot of work but seeing what I have, I don't feel good about leaving any foam unprotected below grade. The foam glass is nice, but pretty darned expensive.

On my own house, I initially protected the slab edge foam with aluminum flashing. It rotted away after 10 years or so - acidic soil, I think. I replaced some of the deteriorated foam with slabs of AAC block. Not as insulating (it does some) but seems like a long lasting product. I used thinset to stick it to my slab, then stuccoed it.


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